Question to the Baader Mark V / refractor folks: Anyone out there who compared the Baader 2" 1.8x Glasspath Corrector (for refractors) with A-P's BARCON?I would be interested in your experiences and which of both would be the better choice for a 150mm f/7.3 APO refractor, when combined with Baader's T-2 #01A mirror diagonal or #01B prism diagonal, and the Mark V binoviewer. Thanks in advance!

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Great question! That thing puzzles me - for refractors - surely it would bottom out in a diagonal?

Johnny,

The Baader 2" 1.8x Glasspath Corrector for refractors is to be placed in front of the diagonal. There is no risk of bottoming out the diagonal, this GPC has a male T-2 thread at one end, which screws directly into Baader's #01A mirror diagonal or #01B prism diagonal (or may be directly connected with the Mark V for straight through viewing). The long 2" diameter body of this 1.8x GPC has to be inserted over the full length into the 2" eyepiece holder of the telescope. I did not find detailed tech spec about this 2" 1.8x refractor GPC on Baader's webpage, but it is pictured and offered together with Baader's 1.25x/1.7x/2.6x GPC's, see here. Baader offers it for EUR 180,00.

The description says that the optical distance is very important and it should only be used with their T2 mirror diagonal - not another diagonal nor without one. Sounds like the Barcon is going to be more universally applicable.

Some notes on "w/o GPC": Personally, I seldomly use Baader's small screw-in type GPC's (1.25x/1.7x/2.6x) in the Mark V bino. Instead, an empty GPC frame without glass element is inserted in the Mark V most of the time. The black finish of the empty GPC frame covers the chrome ring inside the Mark V connector and helps to avoid stray light. I do most of my high-power lunar and planetary observations with a Baader FFC / Mark V combo and had, with inserted empty GPC frame, no stray light issues.

With regard to your question on the GPC frame without lens cell: Unfortunately, no one sells them, neither Baader itself. So I started with a regular spare GPC (one may choose either of the three GPC models). Removal of the cemented 2-element lens cell from the GPC frame is an easy task: Please look at this GPC picture. On top you see a retaining ring with two very small indents for a spanner wrench. No great force is required to loosen that ring, I just used a simple, appropriate-sized needle to loosen and unscrew it. With retaining ring removed, the GPC lens cell may be removed without problems, and stored in a safe place. In order to not lose that thiny retaining ring, I suggest to reinstall it after removal of the lens. Done!

The result is an empty black GPC frame which effectively covers that bright chromed ring on the inside of the Mark V quick changer connection. Covering that chromed ring is highly recommended, because depending on how steep or flat the light cone enters at that connection point, the exposed chromed ring might introduce heavy reflections, as reported elsewhere on this CN forum.

The GPC modification causes no harm to any parts, the empty GPC frame may be retrofitted at anytime, if required.

If I remember correctly, the main reason Roland recommends using a GPC is that installing a barlow/GPC ahead of the binoviewer reduces aberrations in the image caused by the binoviewer (the prisms) itself. So, I'm wondering... if you have used the Mark V without any GPC, but with the empty GPC frame installed, and without any other barlow... have you detected any strange aberrations?

If I remember correctly, the main reason Roland recommends using a GPC is that installing a barlow/GPC ahead of the binoviewer reduces aberrations in the image caused by the binoviewer (the prisms) itself. So, I'm wondering... if you have used the Mark V without any GPC, but with the empty GPC frame installed, and without any other barlow... have you detected any strange aberrations?

I think it depends on the optical system (refractor or reflector) and the entire optical chain. There is no general rule, if the Mark V without inserted GPC works for everyone. I do most of my lunar and planetary observations with a TOA-150B f/7.3 APO refractor. Most of the time, Baader's FFC is used for achieving the wanted long focal lengths, set for resulting focal ratios between f/30-f/33. I can only speak for myself, but in my particular case, it works well without additional GPC in the Mark V. The image quality leaves nothing to be desired: at 400x, moon limb is color-free, background is deep black as velvet, contrast is tack sharp and I have no reflection or stray light issues.

I intend to use my still new and unused Baader 2" 1.8x GPC for refractors as well without second GPC in the Mark V. I hope it works fine at the resulting f/13 focal ratio. Yes, it is true, I have to confess that I never liked the idea of two GPC's in the same optical chain.

I think it depends on the optical system (refractor or reflector) and the entire optical chain. There is no general rule, if the Mark V without inserted GPC works for everyone. I do most of my lunar and planetary observations with a TOA-150B f/7.3 APO refractor. Most of the time, Baader's FFC is used for achieving the wanted long focal lengths, set for resulting focal ratios between f/30-f/33. I can only speak for myself, but in my particular case, it works well without additional GPC in the Mark V. The image quality leaves nothing to be desired: at 400x, moon limb is color-free, background is deep black as velvet, contrast is tack sharp and I have no reflection or stray light issues.

Thanks very much for the info! I plan to be using a 180mm F/7 refractor, so it's a pretty similar system I think. Your empty GPC frame idea is really interesting to me, because 1. I could use my BARCON barlow with the empty frame, and it would provide another range of magnifications without fear of aberrations because the BARCON would help to eliminate them, like a GPC usually does (right?), and 2. Most interesting because using an empty frame with NO barlow or GPC would result in a really low mag, wide field, more so than with the 1.25GPC installed of course... so with 24pans, Mark V, and no GPC or barlow, WOW! If there are no bad aberrations. (And if my analysis makes sense? LOL.) The empty frame makes it really worth testing and trying. Awesome invention.

2. Most interesting because using an empty frame with NO barlow or GPC would result in a really low mag, wide field, more so than with the 1.25GPC installed of course... so with 24pans, Mark V, and no GPC or barlow, WOW!

Most telescopes will not allow such a bino setup without any GPC, they will not come to focus (not enough focus inward travel). In the TOA-150B f/7.3 it works well and delivers 46x super wide field views of the milky way! Stars are pinpoint sharp, no image issues. But I found eye placement is very critical with this particular setup: As long as eyes are placed exactly over the optical axis of each eyepiece, image quality is perfect over the entire field. Changing eye position from optical axis causes slight colors around brighter stars. Of course, the eyepiece cups help a lot to retain the eyes at correctly centered position. Despite the critical eye placement, it is a workable and highly enjoyable bino setup.

I think it is well worth to give it a try in your coming 180mm f/7 refractor, to see if it comes to focus with that setup, too.

Most telescopes will not allow such a setup, they will not come to focus (not enough focus inward travel). In the TOA-150B f/7.3 it works well and delivers 46x super wide field views of the milky way! Stars are pinpoint sharp, no image issues. But I found eye placement is very critical with this particular setup: As long as eyes are placed exactly over the optical axis of each eyepiece, image quality is perfect over the entire field. Changing eye position from optical axis causes slight colors around brighter stars. Of course, the eyepiece cups help a lot to retain the eyes at right position. Despite the critical eye placement, it is a workable and highly enjoyable setup.

That's interesting, about the potential lack of enough inward focus, and critical eye placement. I'll have to try it, the potential of being able to see a significantly wider field is too good to not try it!! I had considered it with my last refractor, but that chrome ring stopped me... your invention eliminates that problem. Cool.

That's interesting, about the potential lack of enough inward focus, and critical eye placement. I'll have to try it, the potential of being able to see a significantly wider field is too good to not try it!! I had considered it with my last refractor, but that chrome ring stopped me... your invention eliminates that problem. Cool.

Chrome ring or not, I've used this exact setup with my TEC 140, Meade 102ED and Astro-Tech AT80EDTA with no discernible issues. I'll have to try it with a lens-less GPC.

I just re-read this older but still very interesting discussion.
According your very helpful measurements, the BARCON (combined with above mentioned eyepieces and Mark V bino without GPC in place) yields magnification factors from 2.52x to 2.59x.

Please allow me a follow-up question with regard to A-P's new 2" Advanced Convertible Barlow(BARADV), just in case that you have already done similar measurements with it:
With all other parts unchanged (see above), but with BARADV replacing the BARCON, how will the BARADV change above magnification figures?
If you did include any Brandon and/or Tak LE-eyepiece (in the focal length range 7.5mm-12.5mm) in your BARADV / Mark V (witout GPC) measurements, your info would be highly appreciated as well.

There is an alternative to covering the chrome ring of the MkV with an empty GPC. The black plastic disk TeleVue supplied with their Radian eyepieces to help center the observer's eye - I think it is also supplied with the Type 4 Naglers - is just the right size to fit into the quick change adapter of the MkV. However, the existing hole in the disk is too small and needs to be enlarged. To enlarge the hole I used a 1-inch Forstner bit and a drill press. Simply tape the disk securely to a scrap piece of wood and then drill out the larger hole. Make sure the Forstner bit is centered on the existing hole. It takes only a few seconds. The Forstner bit will cut the plastic easily. The disk can now be fitted into the quick change adapter and it will cover the chrome ring of the MkV completely.

John, on my personal Mark V bino, I use an empty GPC frame (without any lens element) which covers the bright chromed ring. Your method is a great alternative to anyone out there who wants to use the Mark V without GPC, and wants to avoid buying a standard GPC in order to remove its lens element as I did.